A central problem in consideration of antifertility compounds acting upon the seminiferous epithelium is a determination of whether they act only upon the supporting cells and stem cells, or whether they pass the blood-testis barrier and affect the meiotic and post-meiotic germ cells. The proposed research will explore this question of localization for certain labelled compounds by radioautography; scintillation counting of germ cells isolated by unit gravity sedimentation; and analysis of retetestis fluid. In other studies efforts will be made to separate human x and y spermatozoa and to compare their ultrastructure and assess their relative efficiency in swimming in cervical mucus and other solutions of controlled viscosity.